Constant temperature hot-wire anemometers are often used to measure fluid velocity based on the amount of heat convected away by a fluid passing over a wire heated to a constant temperature. The amount of heat lost due to convection is a function of the fluid velocity passing over the filament. Constant temperature hot-wire anemometers, or CTAs, hold the temperature of a heated filament constant, and use empirical data, mathematical algorithms, or both to calculate the flow rate of a fluid based on the energy used to keep the filament at the constant temperature. Because filament temperature is related to the electrical resistance of filament, the CTA operates to maintain a constant resistance of the filament. Metals used to fabricate suitable filaments have resistivity coefficients on the order of 0.1 percent per degree Celsius, and thus a high degree of accuracy is needed for measuring the actual resistance of the filament.
One medically-related application for anemometers is to measure the inspiration and exhalation flow rates of a patient. Many lung function tests require knowing details on the rate at which air is entering and exiting a patient's lungs. The maximum realistic flow rate range encountered during inspiration and exhalation typically varies between 0 and about 20 liters per second. In this range a filament may have a resistance of only 2.0 ohms. Because the resistance and the resistivity coefficient of the filament are low, even small resistance artifacts can significantly impair measurement accuracy.
In prior art constant temperature hot-wire anemometers, a filament is welded between two pins of a probe. The probe is detachably attached to a cable. The cable communicates with circuitry for calculation of the gas flow rate passing over the filament. There are several problems, however, with the prior art constant temperature anemometer that prevents the acquisition of accurate and precise resistance measurements. For example, there is no way to differentiate between resistivity of the filament and resistivity caused by the cable and any connections between the pins and the circuitry. Any resistance change caused by the cable or the connections will be seen by the circuitry as a change in the resistance of the filament and result in an erroneous gas flow calculation. There are several ways by which resistance errors can be introduced in the prior art constant temperature anemometer probe. These include, for example, changes in ambient temperature, and physical disturbance or movement of the cable and/or connections. Some of these errors cannot be eliminated nor reversed without a complete recalibration of the probe, which can take a considerable amount of time and effort.
Practical considerations require that the probe be designed in such a manner that allows a user to attach and remove the probe from a cable connecting the probe to the unit housing the circuitry such as when the probe is disposable or requires replacement, maintenance, or cleaning. Consequently, cables and connectors are virtually required in all probe designs, thereby insuring the existence of the aforementioned error mechanisms.